1. Field of the Invention
The invention relates generally to ion sources for mass analyzer systems, and more particularly to an electrospray interface.
2. Description of the Prior Art
In its basic form, the electrospray process consists of flowing a solution of the analyte through a capillary tube which is maintained at a high electrical potential with respect to a nearby surface. The solution emerges from a free end of the capillary tube and is dispersed into a fine mist of electrically charged droplets by the potential gradient at the tip of the capillary tube. The size of the droplets formed is determined by a combination of factors including, but not limited to, the solution flow rate, the applied potential and the properties of the solvent. Nebulization may be assisted by directing a co-axial high-velocity gas stream proximate to the free end of the capillary.
Within the ionization chamber, the droplets reduce in size by evaporation of the solvent. Droplet size reduction may also be effected by a microexplosion mechanism caused by the development of high charge density at or near the droplet surface. Eventually, complete evaporation of the solvent is accomplished as the larger droplets become smaller droplets, and the analyte enters the gas phase as an ion.
Under the appropriate conditions, the electrospray resembles a symmetrical cone consisting of a very fine mist (or fog) of droplets (circa 1 μm in diameter.) Excellent sensitivity and ion current stability can be obtained if the fine mist is consistently produced. Unfortunately, the quality of the electrospray is highly dependent on the bulk properties of the analyte solution (e.g., surface tension and conductivity). A poor quality electrospray may contain larger droplets (greater than 10 μm diameter) or a non-dispersed droplet stream. Partially desolvated droplets can pass into a vacuum system, causing sudden increases in pressure and instabilities in the ion current from a mass spectrometer, and reducing sensitivity.
The prior art includes a number of attempts to provide an improved electrospray ion source apparatus that avoids the aforementioned problem associated with incomplete desolvation. Examples of various prior art approaches to addressing the incomplete desolvation problem are disclosed in U.S. Pat. No. 4,935,624 to Henion et al., U.S. Pat. No. 5,157,260 to Mylchreest et al., and U.S. Pat. No. 5,349,186 to Ikonomou et al. However, the prior approaches have been only partially successful at solving the desolvation problem, and some of the approaches are not favored because they create a different set of operational problems.